This invention relates to a bone screw, and, in particular, to a pedicle screw.
Bone screws are typically used in internal fixation to anchor the fixation system to the relevant bone portions. For example, screws can be used in plate or rod systems to treat conditions such as vertebral instability. Overall these systems consist of a longitudinal support, i.e. the plate or rod, and anchoring elements, i.e. screws and/or hooks, for attaching the longitudinal support to the vertebrae. If screws are used, they usually pass through the pedicle and are accordingly referred to as pedicle screws. Because pedicle screws are stably affixed in the bone, they can provide more stability than hooks.
Regardless of the particular application, the design of the screw is very important since the design will have a direct impact on the short term and long term viability of the screw as a means for anchoring fixation systems to bone. In terms of design, the screw is broken up into two major sections, a head portion which, in the case of a pedicle screw, links to the fixation element, and a stem portion (or shaft) which anchors into the bone. The design of the shaft is particularly important in terms of short term and long term viability, with the short term stability dictated solely by mechanical considerations and the long term stability determined by a combination of mechanical (e.g. fatigue strength of the screw) and biological (e.g. bone/screw interface) considerations.
Pedicle screws in particular are subjected to bending and traction stresses. Excessive bending stress may result in xe2x80x9ctraumaticallyxe2x80x9d or xe2x80x9cbiologicallyxe2x80x9d cutting the pedicle screw in the bone and/or screw breakage. Pedicle screw rupture is a mechanical failure caused by fatigue while excessive traction only jeopardizes the bone-screw interface, not the screw itself.
The prior art teaches three basic types of three different thread designs. These are referred to throughout this specification as Types 1, 2, and 3, respectively, and are now summarized.
Type 1: Screws with Cylindrical Threads
The first type of screw is the cylindrical threaded screw. Both the body of the shaft and the threads of the screw are circular-cylindrical. This type of screw offers the highest resistance to extraction by tearing. Moreover, when the screw is rotated back (or backed out) over a distance of less than 10% of its length, the resistance to extraction by tearing is hardly effected. However, this screw design has several shortcomings relative to other designs:
Poor gripping of bone in the absence of careful predrilling;
Uncompensated widening of the entry site by the inward rotation of the screw, which results from lateral pressure exerted by soft tissue; and.
Wear on the thread in the bone during inward rotation (the longer the screw, the greater the wear).
Published European patent application 491,211 and U.S. Pat. No. 2,382,019 both disclose type 1 screws. The disclosed screws have two circular-cylindrical body segments of different body and outside diameters joined by a conical transition segment. The front part of the disclosed screws lack threading. Because of the absence of a thread on the front portion, the screw will provide poor gripping of the bone in the absence of large axial pressures. For brittle, sclerotic bones, the grip of the tip may be so poor that the bone thread will tear loose when attempting to drive the screw through the bone. Published European patent application 669,110 and U.S. Pat. No. 2,382,019 disclose a screw having a threaded front part, but the screw lacks a sequentially multi-stage configuration screw core.
Type 2: Screws With Fully Conical Threads
In these screws, both the body of the shaft and thread are conical. This type of screw offers the advantages of the ability to grip the bone tightly. However, this screw design has the drawback of having a sharp end that can cut into the comparatively soft, spongy, vertebral bone tissue. In addition, when this screw is rotated back, it does not retain the grip the bone as well, thereby increasing the tendency to loosen over time.
Type 3: Screws With Partly Conical Threads
This screw type has a conical body with a thread that is circular-cylindrical. Just as the type 1 screw, this screw is highly resistant to extraction by tearing on account of its cylindrical threads. However, there are several drawbacks to this screw design. First, if the screw is rotated back, the screw grip will likely loosen over time. In addition, although the danger for cutting is lower in this screw design in comparison to the conical screw, there still remains the possibility of cutting. Finally, as a result of the conical body shape and thread shape, the top of the screw shaft portion is wide which can result in bone fissures.
Thus, a need exists for an improved screw design that provides resistance to bending and cutting as well as improved resistance to extraction by tearing and offers good bone gripping independent of bone quality.
The present invention relates to a bone screw, and, in particular, to a pedicle screw. The screw has a head, a front portion having a thread and tapering convexly toward a tip, and a central portion. At least a section of the central portion has a contiguous thread with a substantially constant outside diameter. The threaded section comprises M segments having a body widening toward the head mutually alternating with N segments having a circular-cylindrical cross-section. M and N are integral numbers larger than 0 differing at most by a magnitude of 1, with M greater than 1 and N greater than 2.
In an exemplary embodiment, there are two N segments with the N segment nearer the head larger in diameter than the other N segment and the M segment tapers toward the front portion. The thread of the N segment nearer the head can be thicker than the thread of the other N segment.
The screw can include a non-threaded guide beak that is axially coupled to the tip of the front portion. The screw head can have a substantially frusto-spherical shape. Furthermore, a transition region can be located between the head and the central portion. This region can be threaded and preferably has a circular-cylindrical shape and a diameter that increases from the central portion toward the head. In one embodiment, the transition region thread has inner and outer diameters conically widening from the central portion toward the head. The transition region thread can have the same pitch as the threaded central portion section.
In one embodiment, the central portion section thread is a double thread, with a thread angle between 10xc2x0 and 22xc2x0 and a pitch between 3 and 5 mm. In another embodiment, the central portion section thread is a single thread, with a thread angle between 5xc2x0 and 18xc2x0 and a pitch between 1.5 and 4.0 mm. Regardless of the type of thread, the central portion section thread can be contiguous with the front portion thread. Additionally, the front portion thread and the central portion section thread can have the same pitch and comprise substantially similar number of thread turns.
In one embodiment, the convex taper runs tangentially into the central portion and has a cross sectional radius between 30 and 50 mm. In another embodiment, the front portion terminates in a spherical radius between 0.5 and 3.0 mm.